1. FIELD OF THE INVENTION
The present invention pertains to the field of mechanical locking devices, particularly of the type wherein a rod is axially translatable through a lock housing and having one or more coil springs normally gripping the rod and locking the same to the housing, the rod being releasable by unwinding the coil spring.
2. STATE OF THE PRIOR ART
Considerable inventive activity has been directed towards developing and improving linear friction lock mechanisms. These devices are characterized b one or more coil springs coaxial to the rod. One end of the coil spring is fixed in relation to the lock housing while the opposite end is engaged to a bushing rotatable about the rod. In a normal state the coil spring has an inner diameter somewhat smaller than the rod diameter and thus firmly grips the rod in a friction lock to prevent relative movement of the rod through the housing. A release lever is actuatable, manually or otherwise, for turning the rotatable bushing to momentarily unwinding the coil spring to increase its inside diameter and free the rod for axial translation through the housing. When the release lever is released the coil spring returns to its normal, rod gripping state. An example of a single coil spring friction lock is shown in commonly owned U.S. Pat. No. 4,411,339 issued to Porter on Oct. 25, 1983. Friction locks have found widespread application in adjustable vehicle seat installations. For example, the driver's seat in an automobile is mounted for sliding movement towards and away from the steering wheel on a pair of mounting rails and the seat is normally locked to one of the rails by means of such a friction lock mechanism. When adjustment of the seat is desired, the occupant of the seat manually actuates the release lever, moves the seat to the new desired position and releases the lock lever to secure the seat in the new position.
It has been found that while such locks are generally capable of reliable performance, under certain circumstances the rod may creep through the housing even while being gripped by the locking coil spring. This may happen for example, under conditions of severe vibration or other circumstances where a high axial load is repeatedly applied to the rod. This occurs in single coil locks because there is an inherent degree of asymmetry in the gripping action of the spring which is unwound only at one end. Efforts have been made to overcome this difficulty by providing dual spring lock mechanisms such as disclosed in commonly owned U.S. Pat. No. 4,577,730 issued to Porter on Mar. 25, 1986.
Nevertheless, a continuing need exists for improved linear friction lock mechanisms having positive rod locking characteristics under severe vibration or intermittent load conditions. In particular, there is a need for simple, relatively compact single coil spring friction locks featuring positive fail-safe redundant locking for securing the rod against creep through the coil spring and also to hold the rod in the event of structural failure of the coil spring. Such a redundant lock should be of simple and economical construction, and of uncomplicated operation.